Method of providing VLSI-quality crystalline semiconductor substrates

ABSTRACT

A method is described which makes it possible to use VLSI-quality crystalline semiconductor substrates for the fabrication of the active devices of Active Matrix Flat Panels (AMFPD). The VLSI substrates are provided by arranging a layer of light transparent material in those areas of a semiconductor wafer in which no active device has to be provided, eliminating the semiconductor wafer whereby a transparent wafer is obtained with crystalline semiconductor regions therein and then shaping the transparent wafer into a sized module unit. Several module units can be bonded to a glass substrate and a conductive material is then deposited to make electrical interconnections between the module units. The bonding operation can be performed either at room temperature using a light-transparent glue or at higher temperature using a wafer bonding technique known in the art of Silicon-On-Insulator technology.

FIELD OF THE INVENTION

The present invention relates generally to the fabrication of ActiveMatrix Flat Panel Display (AMFPD) devices, and more particularly, to amethod of providing VLSI-quality crystalline semiconductor substratesfor use in the production of AMFPDs, of which Active Matrix LiquidCrystal Displays (AMLCDs) are given as an example of application.

BACKGROUND OF THE INVENTION

It is widely acknowledged that the most important cause of reduction inyield in high volume production of large area AMFPDs is the high numberof damaged active devices. The main causes for having damaged devicesare the high defect density and also the grain boundaries that exist inthe material which the substrates are made of. These causes placeunavoidable limits on device dimension down-scaling, and therefore oncircuit redundancy, as well as pixel size and density.

AMFPD devices are currently fabricated on amorphous silicon substrates.The use of amorphous silicon results from the fact that it is notpossible to produce crystalline substrates by simply depositing siliconon an amorphous starting material such as glass. Although it is possibleto obtain polycrystalline substrates, their fabrication requires moreexpensive starting materials to be used and/or more processing steps tobe applied.

The standard LCD fabrication has been disclosed by W. C. O'Mara in"Active Matrix Liquid Crystal Displays Part I: Manufacturing Process",Solid State Technology December 1991, pp. 64-69, and statistics for thecauses of failure of assembled panels are reported therein.

In the article "Present and Future Trend of Electron Device Technologyin Flat Panel Display" by T. Uchida, IEEE IEDM Technical Digest 1991,pp. 1.2.1-1.2.6, there is disclosed a formula for the yield of LCDpanels as a function of the defect density, circuit redundancy, pixeldensity, and total area of the panel.

The possibility of having a high quality crystalline semiconductorsubstrate, like the silicon wafers employed in VLSI technology, woulddrastically reduce the surface defect density, with a consequentreduction in the number of damaged devices and thereby a majorimprovement in the manufacturing yield.

SUMMARY OF THE INVENTION

The object of the present invention is a method of providingVLSI-quality crystalline substrates for use in mass production ofAMFPDs.

Another object of the invention is to provide VLSI-quality crystallinesubstrates which are effective to improve the quality of transmissiontype AMFPDs.

These and other objects are attained in accordance with this inventionby a method of providing a VLSI-quality crystalline substrate comprisinga lithographic step to define the substrate areas which are not to beused for implementing active devices and the selected areas are etchedto a certain depth and filled with a transparent material, such assilicon dioxide. After a planarization operation which uncovers theunetched silicon area and provides a flat surface, the wafers nextundergo all the processing steps known per se for realizing the activematrix. Thereafter, the back sides of the wafers are thinned andpolished with the previously deposited silicon dioxide acting as apolish stop. After being sawed to a sized rectangular format, the wafersare aligned on top of a glass substrate with their backsides facing theglass and with the spacing between their edges being smaller than whatthe distance a human eye can resolve. The wafers are permanently bondedto the glass substrate, and a conductive material is then deposited tomake electrical interconnections between the wafer pieces. From then on,the usual AMLCD manufacturing procedures are performed.

The bonding operation can be performed using different techniques. Forinstance, the bonding operation can be performed at room temperatureusing a light-transparent glue, e.g. silicone, or at higher temperatureusing a wafer bonding technique known in the art of silicon-or-insulatortechnology. It is to be noted that when using a wafer bonding technique,the active devices may be realized either before or after the bondingoperation.

This invention makes it possible to provide AMFPDs using VLSI-qualitycrystalline substrates instead of amorphous or polycrystallinesubstrates, thus resulting in higher fabrication yields and deviceshaving superior electrical performance and higher integration densitycapability. In addition, by virtue of providing a light-transparentsubstrate in those areas where no active devices are formed, theinvention allows a high level light transmission through the substratefor each pixel.

Further, this invention has a number of additional advantages infabricating large area AMFPDs. The invention permits modular AMFPDs tobe realized using a plurality of interconnected smaller devices with theconsequent result that the fabrication yield of the device is a functionof the wafer area and not of the total panel area.

In case the active devices are formed before the bonding operation, theinvention allows the smaller modules to be tested and repaired beforebeing used and further allows bonding only those modules in which allthe devices are working. Therefore, the production yield of the completedisplay device only depends on the other fabrication steps. Optimisingthe design of a display device does not depend on the display size, buton the size of the wafers that are used. For any screen size or format,use can be made of a standard processing equipment as used in thefabrication of integrated circuits, thereby decoupling the processingequipment from the size of the display device.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a top view of a portion of semiconductor wafer, in which theindividual pixel active and passive areas are lithographically defined.

FIGS. 2 and 3 show an exemplary way of forming the light-transparentlayer on the semiconductor wafer of FIG. 1.

FIGS. 4 and 5 show an alternative way of forming the light-transparentlayer on the semiconductor wafer of FIG. 1.

FIGS. 6 and 7 show another alternative way of forming thelight-transparent layer on the semiconductor wafer of FIG. 1.

FIGS. 8 to 11 illustrate the subsequent method steps according to theinvention up to the realization of a module unit.

FIGS. 12 and 13 illustrate the modular fabrication of a large LCD deviceby bonding several sawed wafers to a common glass substrate andelectrically interconnecting them.

FIG. 14 is an enlarged view of a portion of the device shown in FIG. 13.

DESCRIPTION OF SPECIFIC EMBODIMENTS

Referring to FIG. 1, there is shown a portion of a semiconductor wafer.The dashed lines identify the individual pixel areas. On the surface ofwafer two types of areas are defined using a standard lithographictechnique: the areas 1 where no active devices have to be provided andareas 2 where active devices are to be formed.

In accordance with this invention, a layer of light-transparent materialis arranged in the areas 1. Referring to FIGS. 2 and 3 there is shown anexemplary way of forming the light-transparent layer in the areas 1. Thesemiconductor material is removed from the areas 1 (FIG. 2) using anetch procedure known per se. The unetched areas 2 are so defined. Alayer 3 of light-transparent material is then deposited on the areas 1,to fill the holes formed by the etch procedure. This can be achieved byperforming, for example, Physical Vapour Deposition (PVD) or ChemicalVapour Deposition (CVD) of silicon dioxide. A planarization operation isthen performed (FIG. 3) to uncover the semiconductor crystalline areas2. The planarization operation can be done, for instance, by ChemicalMechanical Polish (CMP). This CMP technique is disclosed in the article"A New Planarization Technique Using a Combination of RIE and ChemicalMechanical Polish (CMP)" by B. Davari, C. W. Koburger, R. Schulz, J. D.Warnock, T. Furukawa, M. Jost, Y. Taur, W. G. Schwittek, J. K. DeBrosse,M. L. Kerbaugh, and J. L. Mauer, in IEEE IEDM Technical Digest 1989, pp.61-64, the content of which is incorporated herein by reference.

An alternative way of forming the light-transparent layer in the areas 1is illustrated in FIGS. 4 and 5. Following this procedure, a layer 3 oflight-transparent material is first deposited onto the semiconductorwafer (FIG. 4) and through lithography,, the areas 2 are defined whereactive devices are to be provided. The light-transparent material isthen etched in the areas 2 down to the semiconductor substrate 1.Thereafter, the crystalline semiconductor is epitaxially grown in thoseareas 2 until the upper level of the light-transparent layer 3 isreached (FIG. 5).

Another alternative way of providing a layer of light-transparentmaterial in those areas where no active devices have to be provided isillustrated in FIGS. 6 and 7. The procedure uses a semiconductor wafer 1shown in FIG. 6, having a buried light-transparent layer 9, e.g. a knownSIMOX wafer. In the areas where no active devices have to be provided,the top crystalline layer is etched down to the buried light-transparentmaterial 9 and thereafter the etched areas are filled with alight-transparent material 3 (FIG. 7).

The semiconductor wafer with a light-transparent layer 3 arrangedthereon is then ready to undergo all the processing steps of a giventechnology in order to make the active matrix that will switch thepixels. The active devices are made on regions 2. The lines and rows oftheir interconnections are made over regions 3.

Any other processing step used in the standard fabrication of activematrices, e.g. ITO sputtering in order to have a transparent electrodefor each pixel, is also effected at this point of the process.

Next, a mechanical support 4 is attached to the layer 3 by using asacrificial adhesive (FIG. 8).

The wafer 1 is then removed (FIG. 9) using a known thinning andpolishing process, for instance preferential polishing, thelight-transparent material also acting as an etch stop. In this regard,reference may be had to the following literature, the content of whichis incorporated herein by reference: "Novel LSI/SOI Wafer Fabricationusing Device Layer Transfer Technique" by T. Hamaguchi, N. Endo, M.Kimura and M. Nakamae, IEEE IEDM Technical Digest 1985, pp. 688-691, and"Silicon Device Thinning Using Preferential Polishing: Progress inFlatness and Electrical Properties" by S. Wada, S. Takahashi and Y.Hayashi, Semicon. Sci. Technol. 7, Number 1A, (1992) A243-A248.S.

The wafer is now sawed to the size and format of the display beingfabricated. The back side of the sawed and thinned wafer is nowpermanently bonded to a glass substrate 5 (FIG. 10).

The bonding operation can be achieved using a light-transparent glue,e.g. silicone, or by a low temperature variation of a wafer bondingtechnique known in the art of silicon-on-insulator technology.

The wafer bonding technique operating at a high temperature ofapproximately 1000° C. is described in "Silicon-On-Insulator (SOI) byBonding and Etch-Back", by J. B. Lasky, S. R. Stiffler, F. R. White andJ. R. Abernathey IEEE IEDM Technical Digest 1985, pp. 684-687, thecontent of which is incorporated herein by reference.

Many lower temperature (<450° C.) variations of this process have beendeveloped. For example, reference may be had to Leslie A. Field andRichard Muller: "Fusing Silicon Wafers with Low Melting TempereatureGlass" Sensors and Actuators, A21-A23 (1990) 935-938.

Also reference may be had to the following Extended Abstracts, Volume91-2 of the Fall Meeting of The Electrochemical Society, Oct. 13-171991: "Low Temperature Silicon Wafer Bonding for MicromechanicalApplications" by H.-J. Quenzer and W. Benecke, Abstract No. 463, pp.684; "Silicon Nitride as a Dielectric for Low Temperature Direct WaferBonding" by K. Pastor, A. M. Hoff and L. Jastrzebski, Abstract No. 468,pp. 692; "Wafer-Scale Integration Using Restructurable VLSI" by W. P.Eaton, S. Risbud and R. L. Smith, IEEE Computer, April 1992.

When the bonding operation is carried out using a light transparentglue, the wafers must be processed before the bonding operation isperformed because the glue cannot withstand high temperatures. In orderto achieve a high transparency to visible light, the wafer is firstthinned.

When the bonding operation is effected using a wafer bonding technique,the active devices, e.g. thin film transistors (TFTs) can be realizedeither before or after the bonding operation.

Next, referring to FIG. 11, the mechanical support 4 and the adhesiveused to attach the sawed wafer onto it, are removed.

The processed side of the wafer is covered with an orientation film, andfrom then on, the usual LCD production procedures are carried out.

Large display devices can be fabricated in a modular scheme by aligningand bonding several sawed and thinned wafers to a common glass substrateat a spacing smaller than the distance which can be resolved by thehuman eye. FIG. 12 shows an exemplary arrangement of six module units 10on a common glass substrate 5. Electrical connections 6 are then madebetween the module units (FIGS. 13 and 14) whereby they behaveelectrically as a single panel. These connections can be made, forinstance, using restructurable VLSI technology (known per se).

Thereafter, the top side of the device may be covered with anorientation film. The standard LCD production procedures can now becarried out.

I claim:
 1. A method of providing a crystalline semiconductor substratefor use in the production of active matrix liquid crystal displaydevices, comprising the steps of:(a) arranging a layer of lighttransparent material in areas of a semiconductor wafer in which noactive device has to be provided, (b) eliminating said semiconductorwafer whereby a transparent wafer is obtained, said transparent wafercomprising crystalline semiconductor regions, and (c) shaping saidtransparent wafer into a sized module unit.
 2. The method of claim 1,wherein the step of arranging a layer of light transparent material insaid areas of a semiconductor wafer in which no active device has to beprovided comprises:forming areas in a semiconductor wafer in which noactive device has to be provided, and depositing light transparentmaterial in said areas.
 3. The method of claim 1, wherein the step ofarranging a layer of light transparent material in areas of asemiconductor wafer in which no active device has to be providedcomprises:depositing a layer of light transparent material, throughlithography, defining other areas where active devices have to beprovided, etching light transparent material in said areas down to thecrystalline semiconductor substrate, and epitaxially growing acrystalline semiconductor in said areas until an upper level of saidlight transparent material is reached.
 4. The method of claim 1, whereinthe step of eliminating said semiconductor wafer (step b) comprises anoperation comprising thinning and polishing said semiconductor wafer. 5.The method of claim 1, further comprising the step of:forming electricalconnections for interconnecting individual active devices, therebyforming an active matrix.
 6. The method of claim 4, further comprisingthe steps of:bonding several module units to a glass substrate using alight transparent glue, and forming electrical connections forinterconnecting said module units.
 7. The method of claim 2, wherein thestep of eliminating said semiconductor wafer (step b) comprises anoperation comprising thinning and polishing said semiconductor wafer. 8.The method of claim 2, further comprising the step of:forming electricalconnections for interconnecting individual active devices, therebyforming an active matrix.
 9. The method of claim 7, further comprisingthe steps of:bonding several module units to a glass substrate using alight transparent glue, and forming electrical connections forinterconnecting said module units.
 10. The method of claim 3, whereinthe step of eliminating said semiconductor wafer (step b) comprises anoperation comprising thinning and polishing said semiconductor wafer.11. The method of claim 3, further comprising the step of:formingelectrical connections for interconnecting individual active devices,thereby forming an active matrix.
 12. The method of claim 10, furthercomprising the steps of:bonding several module units to a glasssubstrate using a light transparent glue, and forming electricalconnections for interconnecting said module units.